The instant invention relates to chart recorders and more particularly to a thermal chart recorder of the general type which is operative for generating wave form images on a moving chart in response to input signals without the use of moving styli.
Thermal chart recorders which utilize stationary thermal print heads for applying images to thermally sensitive paper charts have generally been found to be highly effective for recording many types of data. In this regard, it has been found that thermal chart recorders are generally capable of generating high resolution wave form images and that because they include a minimum of moving parts they are generally less prone to mechanical failures than other types of chart recorders, such as those which utilize moving styli for applying markings to charts. As a result, thermal chart recorders have replaced conventional moving stylus type recorders in many applications.
In addition to being capable of operating with relatively high levels of efficiency and accuracy, it has been found that most thermal chart recorders are operative for producing markings on charts with substantially less audible noise than traditional stylus type recorders. This is because thermal chart recorders are operative without the use of moving styli which inherently produce scratching sounds as they pass over the surfaces of charts to produce markings thereon. However, while in most instances this feature has been found to be an advantage of thermal chart recorders over stylus-type recorders, it has been found that in certain specific instances it can be a disadvantage. In this connection, it as been found that in some instances operators who have become accustomed to monitoring conventional stylus type chart recorders have learned to rely on the scratching sounds produced by the styli thereof to alert them to significant rapid changes in recorded parameters. As a result, it has been found that in some situations when stylus type recorders have been replaced by thermal recorders the operators thereof have objected to the lack of stylus sounds to alert them to changes in recorded parameters. Accordingly, despite the many advantages of thermal chart recorders, in some situations it has been found that operators have prefered conventional stylus type recorders over thermal recorders.
The instant invention provides a thermal chart recorder which overcomes the above disadvantages of the previously available thermal chart recorders. Specifically, the instant invention provides a thermal chart recorder including means for generating audible sounds which simulate the sounds generated by the moving styli of a stylus type chart recorder. More specifically, the instant invention provides a chart recorder of the general type which is operative for generating wave form images in response to input signals without the use of moving styli. However, in contrast to the previously available chart recorders of this general type, the chart recorder of the instant invention comprises means which is operative for generating audible sound having a volume level which corresponds to the approximate instantaneous absolute value of the rate of change of an input signal to the chart recorder. The chart recorder of the instant invention is preferably embodied as a thermal chart recorder which is responsive to a plurality of input signals for generating a plurality of wave form images and the means for generating audible sound is preferably operative for generating audible sound having a volume level which is proportional to the sum of the instantaneous absolute values of the rates of change of the input signals. Further, the means for generating audible sound is preferably operable for generating sound having a substantially uniform amplitude over a wide frequency range so that the sound generated is essentially white noise. The means for generating audible sound preferably includes means for determining the approximate instantaneous absolute derivative values of the input signals and means which is responsive to the sum of the instantaneous absolute derivative values for producing audible sound having a corresponding volume level. Still further, the means for generating audible sound preferably includes means for filtering the approximate instantaneous absolute derivative values of the input signals for producing filtered derivative signals, and the means for generating audible sound is preferably responsive to the sum of the absolute values of the filtered derivative signals for producing audible sound having a volume level which corresponds thereto.
It has been found that the chart recorder of the instant invention can be effectively utilized to replace traditional stylus-type recorders in applications where operators have become accustomed to relying on the audible sounds generated by the styli of such recorders. Specifically, it has been found that the sound generating means of the chart recorder of the instant invention is capable of producing sounds having volume levels which correspond to the volume levels of the sounds generated by the styli of a stylus type recorder and that as a result, the sound generating means of the chart recorder of the instant invention can be effectively utilized for alerting operators to significant changes in monitored parameters.
Accordingly, it is a primary object of the instant invention to provide a thermal chart recorder which includes means for generating sounds which simulate the sounds produced by the styli of a stylus type recorder.
Another object of the instant invention is to provide a thermal chart recorder which includes means for generating essentially white noise having a volume level which corresponds to the rate of change of one or more input signals to the chart recorder.
Another object of the instant invention is to provide a chart recorder which is operative for generating a plurality of wave form images corresponding to a plurality of input signals and which includes means for generating essentially white noise having a volume level corresponding to the sum of the approximate derivative values of the input signals.
Other objects, features and advantages of the invention shall become apparent as the description thereof proceeds when considered in connection with the accompanying illustrative drawings.